Well you could drag the top right corner of the beginning of each clip say 5 frames to create a transition, then change it to a white dissolve or flash. But probably the easiest way would be to put a solid white background on a lower track, then keep a small gap between each clip (which is obviously on the track above). To multiply the images, put the same clip above the main clip, then lower the opacity to see through it, and experiment with crop and pan. I notice that they've zoomed in slightly, and changed the colour to emphasise. You can also use a mask if you want a specific part of the clip only.

Way OT I know but ... What a horrible video! Those flashes on EVERY top line note played by the piano really detract from what could have been good. Made it far too busy IMO. Editing for editing sake rather than sympathetic to the music.